To add to your fourth point, it should also ideally handle all modern font types and all of their provided features in some relatively simple way. XeTeX/XeLaTeX is great, but that it can’t always do the right thing when any regular TeX document is dropped into it is unfortunate (cf. http://scripts.sil.org/cms/scripts/page.php?item_id=xetex_fa...), and it doesn’t support everything it would in a perfect world.